Cannabis Seeds FAQ - What Are Fertilisers?
In the broad sense of the term, fertilisers are plant food. In some cases, you will need to feed your plant later than others (like if you used enriched soil such as All-Mix or Terra Professional which are packed with slow release nutrients), but all in all, there will come a time when it is necessary to complement the water with some fertiliser.
It is normally divided into Grow, Bloom and Boosters, but depending on brands and kinds you may also encounter 2-part fertilisers (usually for cocos or the Advanced Nutrients line) or 3 part fertilisers (Bio Bizz' organic line or Advanced Nutrients PH Perfect) which may be confusing to choose from at first but as a rule of thumb we suggest using organic nutrients if you're not an experienced grower or do not have the necessary equipment to measure pH and Electrical Conductivity (EC).
Now you know what are fertilisers, but how do you use them?
Based on the Bio Bizz organic line, you would begin with the Grow nutrient (rich in N), then go for the Bloom nutrient (rich in P and K), then compliment the flowering with Top Max (bloom stimulator).
The grower would observe the plant and tell from signs like the presence of flowers to begin feeding with bloom nutrients, or use the nutrient calendars that different brands suggest.
After the use of fertilisers (especially the more advanced mineral ones) it is recommended to perform a flush during the last week of flower before the harvest, since this will clean the plants from mineral residue buildup in its system and help during the smoke to prevent a chemical feeling in the mouth characterised by raspy smoke that can cause unstoppable coughing and other irritations to the throat.
You might also find our FAQ Submission How Do I Harvest My Plant? useful